Audio and video data configuring broadcast programs are encoded in accordance with the moving picture expert group (MPEG) 2 standard into units as prescribed and are digitally broadcast. Recently, an object of a data broadcast associated with a broadcast program is broadcast together with audio and video data configuring the program.
A receiver receives the encoded audio and video data and the data broadcast's object and stores the data and the object to a storage device, and also reads the data and the object from the storage device for reproduction.
FIG. 16 shows a stream of audio and video data encoded by MPEG2, as represented on a time base for display, a stream of the data in file space, and a search table. The audio and video data encoded by MPEG2 has a structure grouped by a unit referred to as “group of pictures (GOP).” A plurality of GOPs are each formed of a B picture, an I picture and a P picture. On the timebase for display a stream 300 has each GOP starting with a B picture, whereas in the file space a stream 400 has each GOP starting with an I picture. This is because decoding video data encoded by MPEG2 can only start at an I picture employing only intra-image compression. At the top of each GOP a sequence header exists, and the top of the header will be a point of random access.
When audio and video data is stored to a storage device, an object of a data broadcast associated with a broadcast program formed of the audio and video data to be stored is also received. Accordingly, to search each program in accordance with an object, a search table 500 is created that is formed of an object of a data broadcast, start and end time stamps indicating a timing, on a timebase for display, of displaying to an external output device the audio and video data of a period of time for which an object of each data broadcast is transmitted, and a random access point corresponding to each start time stamp, as seen in file space (file location of the top of a GOP), and the table is stored to the storage device together with the audio and video data. Thus, search table 500 is formed of an index object, a start time stamp, an end time stamp, and a file location of the top of a GOP.
If a GOP is formed of audio and video data of 0.5 second for 15 frames, then a random access point T in seconds on the timebase for display will be a random access point n in the file space, and a random access point T+0.5 in seconds on the timebase will be a random access point n+1 in the file space, and a random access point T+1.0 in seconds on the timebase will be a random access point n+2 in the file space.
If a data broadcast's object (m) is broadcast from start time stamp T through end time stamp T+5.0 in seconds and an object (m+1) is broadcast from start time stamp T+1.0 through end time stamp T+11.0 in seconds, then object (m) synchronizes with audio and video data starting at random access point n and object (m+1) synchronizes with audio and video data starting at random access point n+2.
As such, if table 500 correlating each object (m), (m+1), . . . broadcast by a data broadcasting service, start and end time stamps indicating a period for which each object (m), (m+1), . . . is broadcast, and a file location of the top of a GOP with each other, is stored to a storage device together with stream 400 provided in file space, each object (m), (m+1), . . . can be used to allow a user to reproduce audio and video data at a location as desired for reproduction.
More specifically, when a user designates object (m), a file location n of the top of a GOP corresponding to object (m) is read from search table 500 and reproduction starts at audio and video data present at the read file location n. This also applies when other objects (m+1), . . . are designated.
With reference to FIG. 17, audio and video data stored in a storage device is searched for and reproduced in accordance with an object transmitted by a data broadcasting service, as will now be described. When reproduction starts, a user designates a broadcast program that the user desires to reproduce, from the designated program's search table any object is selected and an instruction is issued to reproduce the same (step (S)101). More specifically, when the user designates a broadcast program that the user desires to reproduce, the designated program's search table is read from a storage device and displayed on a CRT. The user refers to the displayed table, selects an object that the user desires to reproduce, and instructs reproduction of the selected object.
Once an object has been selected, the selected object's start and end time stamps and a file entry location, or a file location of the top of a GOP, are ensured from the search table (S102). Then, a stream file's entry location corresponding to the ensured file location is searched for (S103). An entry location is detected, and transferring audio and video data starts at the detected location (S104).
The start time stamp read from the search table is compared with an AV stream's time stamp and if the they match, displaying audio and video data starts (S105). Then when AV stream's time stamp matches the end time stamp read from the search table (S105), reproduction operation ends.
Thus a user can search for a scene that the user desires to reproduce, as based on an object of a data broadcast associated with a broadcast program, and can reproduce the retrieved scene. In this case, an object refers for example to a still picture or a caption.
Recently, however, issues associated with protection of copyright of digital content are increasingly discussed and rather than exactly storing to a storage device audio and video data broadcast as digital data, encrypting the data in a prescribed system and storing it to a storage device is considered. When audio and video data is encrypted, the data is divided by a prescribed bit count and encrypted.
As such, the audio and video data is divided by the prescribed bit count independently of the GOP unit, and the location of the top of a GOP does not necessarily match that of the top of the encrypted audio and video data. In contrast, when an object transmitted by a data broadcasting service associated with a broadcast program is designated and audio and video data that a user desires to reproduce is randomly reproduced, the object's start location in the aforementioned system matches the top of the coding unit (the GOP unit), and reproduction of encrypted audio and video data cannot be started at a location indicated by the user for reproduction, i.e., the object's start location.